A turbocharged engine typically has a turbine that is driven by the normally high-pressure exhaust gases of the engine, and hence has its intake connected to the engine exhaust-gas manifold, and a supercharger or compressor that is driven by this exhaust-gas turbine and in turn has its output connected to the engine intake manifold. Thus the energy in the exhaust gases is used to drive the compressor that increases the amount of air fed to the engine so as to increase the fuel/air charge for the engine and thereby increase its torque and horsepower.
The above-described very simple system in reality is not used often, in particular for a motor vehicle where the engine RPM (revolutions per minute) varies very widely. The simple system operates efficiently only in a very narrow range, so that the user can only rely on the increased power given by such a turbocharged engine in this narrow range.
It is standard practice, as described in the commonly assigned and copending application 898,416 filed 20 Apr. 1978 whose entire disclosure is herewith incorporated by reference, to provide a relatively small and fast-acting turbine for the exhaust gases so that even at relatively low engine speed the turbocharger operates with maximum efficiency. A shunt or bypass valve is provided across this turbine and is set up so that it opens above a predetermined engine speed to reduce the exhaust-gas pressure driving the turbine and thereby prevent the turbine from overdriving the supercharger. Such overdriving of the supercharger would lead to the feeding of excessive air to the intake manifold, which would inevitably lead to knocking. In a fuel-injected system where the fuel injector is provided upstream of the supercharger such a system could even result in spontaneous ignition of the fuel-air mixture in the intake manifold, resulting in an explosion that could have disastrous results.
Thus the bypass valve is normally urged into the closed position by a spring, and can be operated by a membrane which is acted on by the pressure of the exhaust gases, as also shown in U.S. Pat. Nos. 3,195,805 and 3,270,951. Thus when the exhaust-gas manifold pressure exceeds a predetermined limit it will open this bypass valve and thereby relieve some of this pressure to prevent the exhaust-gas turbine from operating the supercharger at such a speed that it feeds excessive air to the engine intake manifold.
The operating characteristic of such an engine is normally fairly simple. The amount of torque produced by the engine, which is almost a direct function of the intake-manifold pressure, increases up to the point where the bypass valve opens. At higher RPM the torque increases only slightly, or even decreases slightly up to the maximum rated RPM for the engine. This is due to the fact that once the valve opens the exhaust-manifold pressure only increases slightly, as a result of the use of a biasing spring which inherently becomes somewhat stiffer as it is compressed. Thus the curve for the exhaust-manifold pressure in a standard such system will be effectively constituted by a slightly upwardly inclined generally straight line, whereas the curves for the intake-manifold pressure and engine torque, which are closely related, will be represently by normally level or slightly falling lines, due to the fact that an RPM above the nominal engine RPM will cause the engine efficiency to drop somewhat.